Hydraulic control valves typically have a centering spring device to return the valve spool from an operating position to a neutral position when the actuating force acting on the spool is removed. The springs of the device are assembled with a preload and has a certain rate such that as the spool is shifted from its neutral position the return force of the spring increases. The return force of the centering spring device must be sufficient to overcome the flow forces acting on the spool at the operating position and is generally quite high particularly on large valves rated at 200 to 380 liters per minute. The return force of the centering spring device constitutes a major portion of the force required to shift the spool.
The high spring force has not been a problem with pilot operated valves wherein the force required to overcome the centering spring is provided by pressurized pilot fluid. However, the spring force presents a major problem with the use of direct electrical actuators to position the valve spool. Direct electrical actuators that are required to move the spool against both the flow forces and the return force of the spring would be quite large, consume large quantities of electrical power and generate large amounts of heat.
Thus, it would be desirable to eliminate the effect of the centering spring force during a typical work cycle yet be able to reactivate that force to return the spool to the neutral position when the system is shut down or in the event the electrical power is lost.
The present invention is directed to overcoming one or more of the problems as set forth above.